Re: [Xen-devel] [v3,11/41] mips: reuse asm-generic/barrier.h

["Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>]

On Thu, Jan 14, 2016 at 03:33:40PM -0800, Leonid Yegoshin wrote:
> On 01/14/2016 02:55 PM, Paul E. McKenney wrote:
> >OK, so it looks like Will was asking not about WRC+addr+addr, but instead
> >about WRC+sync+addr.
> (He actually asked twice about this and that too but skip this)

Fair enough!  ;-)

> >I am guessing that the manual's "Older instructions which must be globally
> >performed when the SYNC instruction completes" provides the equivalent
> >of ARM/Power A-cumulativity, which can be thought of as transitivity
> >backwards in time.  This leads me to believe that your smp_mb() needs
> >to use SYNC rather than SYNC_MB, as was the subject of earlier spirited
> >discussion in this thread.
> 
> Don't be fooled here by words "ordered" and "completed" - it is HW
> design items and actually written poorly.
> Just assume that SYNC_MB is absolutely the same as SYNC for any CPU
> and coherent device (besides performance). The difference can be in
> non-coherent devices because SYNC actually tries to make a barrier
> for them too. In some SoCs it is just the same because there is no
> need to barrier a non-coherent device (device register access
> usually strictly ordered... if there is no bridge in between).

So smp_mb() can be SYNC_MB.  However, mb() needs to be SYNC for MMIO
purposes, correct?

> >Suppose you have something like this:
> >...
> >Does your hardware guarantee that it is not possible for all of r0,
> >r1, r2, and r3 to be equal to zero at the end of the test, assuming
> >that a, b, c, and d are all initially zero, and the four functions
> >above run concurrently?
> 
> It is assumed to be so from Arch point of view. HW bugs are
> possible, of course.

Indeed!

> >Another (more academic) case is this one, with x and y initially zero:
> >
> >...
> >Does SYNC_MB() prohibit r1 == 1 && r2 == 0 && r3 == 1 && r4 == 0?
> 
> It is assumed to be so from Arch point of view. HW bugs are
> possible, of course.

Looks to me like smp_mb() can be SYNC_MB, then.

> Note: I am not sure about ANY past MIPS R2 CPU because that stuff is
> implemented some time but nobody made it in Linux kernel (it was
> used by some vendor for non-Linux system). For that reason my patch
> for lightweight SYNCs has an option - implement it or implement a
> generic SYNC. It is possible that some vendor did it in different
> way but nobody knows or test it. But as a minimum - SYNC must be
> implemented in spinlocks/atomics/bitops, in recent P5600 it is
> proven that read can pass write in atomics.
> 
> MIPS R6 is a different story, I verified lightweight SYNCs from the
> beginning and it also should use SYNCs.

So you need to build a different kernel for some types of MIPS systems?
Or do you do boot-time rewriting, like a number of other arches do?

                                                        Thanx, Paul


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